Heating, ventilating, and air-conditioning systems (HVAC systems) use a plurality of ducts to distribute conditioned air throughout buildings. Depending on the temperature in a room or need to adjust the temperature in the room, the HVAC system will adjust the flow of conditioned air into the room.
In the past, the flow rate of the air was monitored using a velocity pressure transducer using a piezo-electric pressure transducer coupled to a pitot tube positioned within the air stream through the duct. One example of such a transducer is illustrated in U.S. Pat. No. 4,817,022, granted on Mar. 28, 1989 to Jornod et al. As illustrated therein, because the signal generated by the piezo-electric transducer is affected by physical changes in the transducer, one feature of the ′022 patent was a method and apparatus for compensating for changes in the physical properties, such as temperature, of the transducer by using a zeroing process to eliminate offset or drift errors that can be caused by the changes in the physical properties of the transducer.
During normal operation, the transducer was exposed to a differential pressure caused by the different locations of ports in the pitot tube. This differential pressure was sensed by applying the different pressures to opposite sides of the piezo-electric pressure transducer and then used to determine the flow rate of the of air. Every so often, the system would “zero” the pressure across the transducer by determining an offset generated by changes in the physical properties of the transducer. To do this, the system would apply substantially equal pressures to both sides of the transducer such that the pressure drop across the transducer was believed to be effectively zero. This zeroing process occurred by opening a valve to fluidly couple the high and low pressure sides of the transducer, such that substantially identical pressures were believed to be exposed to both sides of the transducer. In this substantially zero pressure state, a reading was taken and this was used as an offset value.
Unfortunately, the size of ducts in commercial and residential building has been increasing in cross-section such that the actual speed of the air flowing through the ducts has reduced. Thus, the pressure transducers are being used to sense smaller and smaller pressure values.
However, when the valve opens to take the zero pressure reading air will flow through the valve while the pressure reading is being taken. This airflow causes a residual pressure drop that introduces an error in the zero pressure measurement. Because the transducers can be employed to measure velocity pressures as low as 0.004 in.WC (1 Pa), this error can be significant.
Therefore, there is a need in the art to eliminate or even further reduce any potential error in the zero pressure measurement.